$10l + 4m - n - 2 = 10m - 9n + 2$ Solve for $l$.
Combine constant terms on the right. $10l + 4m - n - {2} = 10m - 9n + {2}$ $10l + 4m - n = 10m - 9n + {4}$ Combine $n$ terms on the right. $10l + 4m - {n} = 10m - {9n} + 4$ $10l + 4m = 10m - {8n} + 4$ Combine $m$ terms on the right. $10l + {4m} = {10m} - 8n + 4$ $10l = {6m} - 8n + 4$ Isolate $l$ ${10}l = 6m - 8n + 4$ $l = \dfrac{ 6m - 8n + 4 }{ {10} }$ All of these terms are divisible by $2$ $l = \dfrac{ {3}m - {4}n + {2} }{ {5} }$